A bisque schedule to cone 06 that does not blow up thick work

Bisque firing is not a warm-up for the glaze firing; it is the firing that decides whether a piece survives at all. Water and organic material have to leave the clay slowly, and both of those things happen below 700 degrees Celsius. A schedule that climbs at 100 degrees per hour to 600, then 150 to cone 06, and holds for ten minutes handles everything from a 4 mm cup to a 20 mm sculpture.

Candling: the step almost everyone rushes

From 60 to 120 degrees Celsius, physically adsorbed water leaves the clay. For a 4 mm wall this takes an hour; for a 20 mm wall it takes six. If the climb through this range is too fast, the water turns to steam inside the wall and the piece explodes — usually between 200 and 300 degrees, well before anything looks like it is happening.

The safe rule is one hour of hold for every 5 mm of wall thickness, up to 12 hours for very thick work, with the lid propped and the top spy hole open so the vapour has somewhere to go.

Burn-out: ventilation matters more than speed

Between 300 and 600 degrees, organic material in the clay — and any paper, wax or fibre the maker added — burns off. This stage needs oxygen, which means the kiln must be vented. A kiln fired closed through this range produces black-cored ware: the carbon has nowhere to go so it stays in the clay, and the piece looks grey in the break and takes glaze badly.

Climb at 100 to 150 degrees per hour through this range with the bottom vent open and the lid cracked a centimetre until 600 degrees. Some potters hold at 600 for 20 minutes to make sure the carbon is fully gone; it costs 20 minutes and prevents a whole class of glaze faults.

The final climb and the soak

Above 600 the risk is thermal shock rather than explosion, so the schedule can speed up: 150 to 200 degrees per hour to cone 06 at 999 degrees Celsius. Hold for 10 minutes at the end so the whole kiln equalises — without a soak the top shelf will read two cones hotter than the bottom and you will wonder why the same glaze looks different on two pots from the same firing.

Place witness cones on the bottom, middle and top shelves, not just one in the spy hole. The spy-hole cone tells you what the top of the kiln is doing and nothing else.

Cooling: the part that gets ignored

Do not open the kiln above 150 degrees Celsius. The quartz inversion at 573 degrees is reversible, and a bisqued piece cooled quickly through it will develop dunting cracks that only appear in the glaze firing. Let the kiln cool naturally with the lid shut until the pyrometer reads below 100 degrees, then crack the lid for an hour before unloading.

A typical cone 06 bisque takes 8 to 10 hours of firing and 10 to 14 hours of cooling. Loading a kiln at 5 pm and unloading the next evening is realistic; unloading the next morning is not, and the pots that get unloaded early are the ones with the fine cracks.

Steps

1. Load the kiln with witness cones on the bottom, middle and top shelves.
2. Candle from 60 to 120 degrees at 60 degrees per hour, holding one hour per 5 mm of the thickest wall in the load.
3. Open the bottom vent and crack the lid a centimetre for the burn-out stage.
4. Climb at 100 to 150 degrees per hour from 300 to 600 degrees with the kiln vented.
5. Optionally hold at 600 degrees for 20 minutes to ensure the carbon is fully burned out.
6. Climb at 150 to 200 degrees per hour from 600 to cone 06 at 999 degrees Celsius.
7. Soak for 10 minutes at the end and check the witness cones across all three shelves.
8. Shut the kiln and let it cool to below 100 degrees before cracking the lid, then wait an hour before unloading.

Recipe and schedule numbers here were recorded on the workshop’s own kiln. A different kiln, a different thermocouple position or a different clay batch will move the result, so treat every firing schedule as a starting point and keep your own log beside it.

← All Techniques